1. Field of the Invention
This invention relates generally to a system and method for reducing disturbances on a high voltage bus in a fuel cell system including a high voltage battery and, more particularly, to a system and method for reducing disturbances on a high voltage bus in a fuel cell system including a high voltage battery, where the system also includes a DC/DC boost converter that calculates a time derivative of the bus voltage to adjust a stack current set-point.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source in addition to the fuel cell stack, such as a high voltage DC battery or an ultracapacitor. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to an electrical traction motor through a DC high voltage electrical bus for vehicle operation. The battery provides supplemental power to the electrical bus during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge the battery or ultracapacitor at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery or ultracapacitor.
In the hybrid vehicle discussed above, a bi-directional DC/DC converter is sometimes employed to match the battery voltage to the voltage of the fuel cell stack.
Typically, each high voltage component in the electrical architecture of a fuel cell system includes an input capacitance that acts as a low pass filter to filter the disturbances on the high voltage bus to reduce their affect. Generally, in order to reduce the disturbances on the high voltage bus to a suitable level, a relatively high capacitance is needed in each component coupled to the high voltage bus, which increases the cost of the system, increases the size of the system, increases the weight of the system, etc. Further, higher capacitances present a potential problem when the system is shut down because those capacitances may need to be discharged. Therefore, it is desirable to reduce the size of the capacitance in the high voltage electrical architecture of a fuel cell system to be as small as possible.
In one fuel cell system design, the electric traction motors and other miscellaneous loads operate on the voltage determined by the fuel cell stack. The difference between the fuel cell stack voltage and the battery voltage is provided by a DC/DC converter that couples the battery to a high voltage bus connecting the fuel cell stack to other high voltage components. Any disturbances on the bus will be filtered by the input capacitance of the various devices, which are rather small, and the inherent capacitance of the fuel cell stack, which is relatively large. The converter will not pass the disturbances to the battery because of its inherent behavior and the various system set-points from the high level system controller will be too slow to react to the disturbances.
Other fuel cell system designs operate on the voltage determined by the battery, where those components are electrically coupled to the high voltage bus between the DC/DC converter and the high voltage battery. The DC/DC converter connects the fuel cell stack to the high voltage bus. In this design, any disturbances on the bus will also be filtered by the input capacitance of the various devices and the inherent capacitance of the battery. However, the high ohmic resistance of the battery at very low temperatures decouples the battery's capacitance from the high voltage bus and reduces its ability to provide the dampening effect of the battery. Further, the high inductance of the cables coupled to the battery or the battery internal current routing further suppresses any damping effect. The converter will not pass the disturbances from the fuel cell stack because of its inherent behavior and the various system set-points from the high level system controller will be too slow to react to the disturbances. Thus, these disturbances would drive big voltage fluctuations on the high voltage bus.